Fossil fuels are typically combusted in industry to produce heat and/or electricity. The combustion results in the production of a stream of flue gas which contains carbon dioxide and other components. In addition, other sources of waste gas streams containing carbon dioxide, which may be produced by industry, include landfill gas, blast furnace gas and off gas from an electric arc bauxite reduction furnace.
Carbon dioxide has been identified as a green house gas. Accordingly, the amount of carbon dioxide emitted with flue gases from an industrial plant are subject to regulation in many jurisdictions. Therefore, waste gas streams, prior to being vented to the atmosphere, typically need to be treated to control the amount of carbon dioxide that is emitted to the atmosphere.
Techniques for separating carbon dioxide from a gas mixture are known. These include the use of regenerable absorbents, cryogenic techniques and membrane techniques. However, each of these technologies is energy intensive when applied to capturing carbon dioxide from a gas mixture. Accordingly, additional energy, which may well be obtained from burning fossil fuels, is required to operate the carbon dioxide capture process. Accordingly, the operating of the carbon dioxide capture process may result in the generation of additional flue gas that must be treated.
Various amine absorbents, which are sometimes referred to as solvents, are well known for use in removing carbon dioxide from flue gas. Factors which influence the economics of a carbon dioxide capture process utilizing amine-base solvents include the liquid to gas ratio (L/G), the regeneration steam requirement and the amine loss rate. The liquid to gas ratio is a ratio of the liquid flow rate (i.e. the flow rate of the absorbent through, e.g., an absorption column) to the gas flow rate (e.g. the flow rate of gas through an absorption column countercurrent to the absorbent). Accordingly, the L/G ratio is a measure of the moles of amine circulated per mole of carbon dioxide in the inlet gas stream. Therefore, the L/G ratio determines the size of the liquid side equipment and pumping power that is required to obtain a particular level of carbon dioxide removal. The regeneration steam requirement relates to the amount of steam that is required to regenerate the amine absorbent. The larger the L/G ratio and the steam required per volume of CO2 rich absorbent, the more energy must be provided to operate the process, to obtain a particular level of carbon dioxide removal. The amine loss rate relates to the extent to which the amine is degraded or lost by volatilization into the treated gas stream, and needs to be replaced. Accordingly, various different absorbents and combinations of absorbents have been proposed.
Primary and secondary amines, such as monoethanolamine (MEA) and diethanolamine (DEA) are very reactive with CO2 and are therefore able to effect a high volume of carbon dioxide removal at a fast rate. Primary and secondary amines however have a limitation in that their maximum carbon dioxide loading capacity, based upon stoichiometry, is at best about 0.5 mole CO2/mole of amine functionality. Further, amines, which form stable carbamates, e.g. strong primary amines, are difficult and energetically expensive to regenerate to low CO2 content in the lean amine, so that the delta loading is often undesirably small per amine functionality. MEA for example has a delta loading of about 0.25 moles CO2/mole amine. Tertiary amines, however, which are thermally and chemically more stable, such as methyldiethanolamine (MDEA), have an equilibrium carbon dioxide loading capacity that approaches 1.0 mole CO2/mole amine. Furthermore stripping carbon dioxide from tertiary amines of moderate pKa, e.g. pKa=8.0-9.5, typically requires substantially less energy input than is required to strip carbon dioxide from primary and secondary amines, such as MEA or DEA.
Accordingly, both primary and secondary amines, as well as tertiary amines, have properties which make them desirable for use in acid gas capture. However, they each have disadvantages. Accordingly, it has been disclosed to use primary and secondary amines as activators for tertiary amines (see for example U.S. Pat. No. 5,700,437; U.S. Pat. No. 5,277,885 and WO2005/087349) in order to overcome their major disadvantage of slow reactivity with CO2.